OK -- I've just started fermentation on my fourth batch, and I've consistently had a low O.G. on all four batches I've done.

The first three i did were Brewer's Best kits, and this last one is a porter that I put together using the recipe generator on BeerTools. But all four times, my O.G. has consistently read about .020 BELOW what was expected. I've calibrated my hydrometer, so I'm pretty sure that's not it, and my F.G. is always where it's supposed to be.

I'm extract brewing, beginning with 2-3 gallons, which boils down by about a gallon, and I top off with water to 5 gallons just prior to taking the O.G reading, correcting for any temp differences.

is the water you add at the end thoroughly mixed into the wort? the heavy wort sits at the bottom of the carboy, so your readings might be diluted and thus pretty low. (look for a color gradient to see if there is stratification.)

i always try to top off either right before the end of the boil (so it mixes by boiling) or add warm water. cold water doesn't mix well with the warm, dense wort; warm water will mix easier.

When you top off with water are you over diluting? Have you validated the number of gallons in your fermenter? Sometimes brewers try to stretch a 5 gallon batch into a 5.5 gallon which will result in a lower OG.

I haven't validated the markings on the side of the fermenter, and as it's full right now, I'll have to wait until I'm done fermenting.

And I usually cool the wort, transfer from my boil pot into the fermenter, and then top off with water, so that may be my problem as well. I usually only boil 2-3 gallons because my stove has a hard time heating more water to boiling, but next time I'll try topping off to 5 gallons right at the end of the boil to see what happens.

Do you suggest I just go with predicted O.G. for logging/ABV purposes?

I usually record both the predicted and actual gravity readings. If there is a big difference in the two, I begin the toubleshooting process to see if it was me and my proceedures or the recipe. I really don't bother calculating the actual ABV. My top few lines in my notes look something like this.

akueck wrote:is the water you add at the end thoroughly mixed into the wort? the heavy wort sits at the bottom of the carboy, so your readings might be diluted and thus pretty low. (look for a color gradient to see if there is stratification.)

If you have diluted to the expected volume, mixing is almost always the the number one reason for not hitting your gravity. This assumes the kit is correctly predicting the gravity.